In general, a laminated glass has a structure obtained by interposing an interlayer film for laminated glass (hereinafter also referred to simply as an interlayer film) including a polyvinyl acetal resin such as polyvinyl butyral resin plasticized by a plasticizer between at least a pair of glass sheets, and integrating them. The laminated glass having such a structure is excellent in safety because fragments of glass hardly scatter when broken by an exterior impact, and therefore it is widely used as a window glass for vehicles such as automobiles and aircrafts, and for buildings.
However, such a laminated glass including an interlayer film was excellent in safety, but was inferior in heat shield properties. In general, infrared radiation having a wavelength of 780 nm or more, which is longer than that of visible light, is referred to as heat ray because, inspite of its small energy amount such as about 10% as compared with ultraviolet radiation, it has a large thermal action and is released as heat to cause temperature rise when absorbed by a substance. A laminated glass capable of effectively shielding this heat ray has been required. When it becomes possible to shield infrared radiations having a large thermal action among light rays incident upon automotive front and side glasses, heat shield properties are enhanced and thus temperature rise in the automobile can be suppressed. Since the area of the automotive glass portion tends to increase, recently, it has become necessary to enhance heat shield properties of the laminated glass, thereby imparting a heat ray shield function to the glass opening portion.
As the laminated glass having enhanced heat shield properties, for example, there has been known a laminated glass including an interlayer film having a transparent resin mixed with a plasticizer containing tin-doped indium oxide fine particles (hereinafter also referred to as ITO fine particles) to a transparent resin (see Patent Document 1: Japanese Patent No. 3,040,681). This publication discloses, as the interlayer film for laminated glass, an interlayer film obtained by mixing ITO fine particles having a particle size limited to 0.1 μm or less so as not to impair transparency, an anionic surfactant and phthalic acid di-2-ethylhexyl as a plasticizer to prepare a dispersion of ITO fine particles containing ITO fine particles dispersed therein, kneading the dispersion with a polyvinyl butyral resin, and forming the kneaded mixture into a film.
As an interlayer film composition for laminated glass having heat shield properties, there has been known a composition obtained by mixing a dispersion containing ITO fine particles, a higher fatty acid ester, and a plasticizer with a resin (see Patent Document 2: Japanese Patent Application, First Publication No. 2001-233643). In the case of this interlayer film composition, the higher fatty acid ester such as polyglycerin fatty acid ester is added so as to enhance dispersibility of ITO fine particles.
However, a conventional interlayer film for laminated glass composition, or a dispersion of ITO fine particles used for the interlayer film composition may be inferior in transparency because clouding occurs at a certain angle, inspite of the same haze value as an indicator of transparency. Also, there is a problem in that, when using a conventional dispersant in the case of dispersing ITO fine particles in the plasticizer, it becomes difficult to control the degree of adhesion at the interface between the glass and the interlayer film of the laminated glass. Also, there is a problem in that it becomes difficult to control a variation in the degree of adhesion between the glass and the interlayer film due to a change in moisture of the interlayer film. Furthermore, there is a problem in that, when the dispersion of ITO fine particles is diluted with a plasticizer for an interlayer film, ITO fine particles are converted into agglomerated particles due to poor dispersion, that is, so-called solvent shock phenomenon arises and thus transparency is lowered.
There have been known a composition obtained by adding triethylene glycol di-2-hexanoate (3GO) as a plasticizer to a solution containing ITO fine particles dispersed in polyphosphate ester and acetylacetone (see Patent Document 3: Japanese Patent Application, First Publication No. 2002-293583) and a composition obtained by further mixing the composition with 2-ethylhexanoic acid (see Patent Document 4: Japanese Patent Application, First Publication No. 2001-302289). However, all of these compositions have a drawback in that they are free of alcohols and have high hydrophobicity, and thus ITO fine particles are inferior in affinity with the solution and solvent shock may arise. Also, there is a drawback in that dispersion proprety drastically vary depending on the kind of the plasticizer for the interlayer film.